Low bit-rate voice coders are currently being investigated as a means of reducing the amount of information required for transmission or storage. One such voice coder is a digital sub-band coder, which operates on speech segments to divide a speech signal into several spectral sub-bands to determine where significant speech energy resides. Typically, a predetermined number of digital bits are allocated among those significant sub-bands to code the spectral information for transmission.
In a context of a radio frequency communication system, only the essential spectral information is transmitted to a receiver, which reconstructs or synthesizes the speech signal by routing the essential spectral information to reconstruction sub-band filters. In an attempt to avoid "hollow" sounding speech or artifacts caused by not transmitting all of the spectral bands, conventional designers of sub-band coders pass noise through any unexcited sub-band filters in an attempt to perceptually improve the audio quality of the synthesized speech. The filtered noise signals are combined with the transmitted spectral information to generate a more natural sounding speech signal. The amount of noise added in each spectral band is usually scaled in proportion to the amount of speech energy originally present in the corresponding band.
Of course, other speech coders exist, which produce "essential" spectral information, such as, for example, linear predictive coders. Nevertheless, all such speech coders (processors) operate at a fundamental level by processing an input signal to reduce or minimize the spectral information needed to reconstruct or synthesize a speech signal. These speech tecnhiques also beneficially utilize an added filtered or processed noise signal to provide perceptually improved quality synthesized speech.
The current trend in low bit-rate coders provides for further reduction in the amount of speech information that is transmitted. Thus, more reliance must be placed upon other processing techniques to adequately reconstruct a natural sounding speech signal While processed noise signals may be appropriate in some cases, a need exists to provide a technique to achieve high quality synthesized speech signal suitable for use with low bit-rate coders and voice synthesizers.